There are known rotary cutting-tools including a cup-shaped cutting bit mounted on a holder journalled in the housing of the cutting-tool in bearings. However, their common drawback is their insufficient rigidity. To enhance the rigidity, it is necessary to incorporate in the cutting-tool bearings of a high load-bearing capacity and high durability, which bearings are of considerable dimensions, whereby the size of the housing with aid of which the cutting-tool is secured in the tool carrier is increased correspondingly. Therefore, alongside with the enhanced rigidity of the cutting-tool, the amount of metal therein is increased, and its compact size is affected. The compactness of the cutting-tool, however, is of particular importance in such machining techniques as milling and boring.
There is further known a rotary cutting-tool (see, for example, the SU Inventor's Certificate No. 222,120) featuring a compact structure and including a stepped-diameter arbor with bearings supporting a holder with a cup-shaped cutting element or bit.
The holder is made in the form of an outer sleeve having its internal tapering surfaces engaging rolling bodies (either balls or rollers) arranged coaxially with respect to said arbor, the latter having a tail portion secured in the tool head or holder.
A disadvantage of this known cutting-tool disclosed in the SU Inventor's Certificate No. 222,120 is its insufficient rigidity affected, first and foremost, by the fact that the bearing means supporting the holder are mounted on the cantilever portion of the arbor. The same fact would not enable to have the optimum spacing of the bearing means, which would have provided for adequate rigidity and durability of the bearing assembly. To reduce the cantilever portion of the arbor, the bearings have to be arranged practically next to each other.
The insufficient rigidity of the cutting-tool results in the cutting edge being urged away from the surface being worked in the course of the machining operation, and in cases when the machining allowance is non-uniform, e.g. in cases of radial runout of the workpiece, the extent of this urging-away is likewise non-uniform, which drastically affects the finishing quality and the machining accuracy. Moreover, the insufficient rigidity of the cutting-tool brings about inadequate vibration-resistance of the machining process, as a whole, which curbs down the productivity and affects the stability of the cutting-tool, the latter being excessively susceptible to varying cutting conditions. On the other hand, the range of the variation of the cutting duty whereat the process is conducted without vibration is exceedingly narrow, while the probability of the generation of vibration is high. With vibration generated, not only is the accuracy and surface-finishing quality of the machining operation affected, but the stability of the cutting-tool sharply declines, and its service life is curtailed, with intensive crumbling of the cutting edge of the tool. Thus, the assets of the cutting-tool of the prior art, as far as the high productivity, surface-finishing quality and machining accuracy are concerned, are definitely limited.
To the abovediscussed drawbacks of the cutting-tool of the prior art one has to add the fact that its structure has no provisions for positive withdrawal of heat from the cutting portion and bearings heating up in operation. The most primitive cooling technique alone is available, namely, the feed of a cooling fluid directly onto the cutting bit. However, this technique is of but limited applicability on account of its poor sanitary standards and the intensification of the crumbling of the hard-alloy cutting portion of the cutting-tool.